1. Field of the Invention
This invention relates to a Doppler velocimeter which detects changes in the speed of a moving object (a solid, a fluid, or the like, hereinafter termed an "object to be measured") in a noncontact state.
2. Description of the Related Art
Laser Doppler velocimeters detect a moving speed of an object to be measured utilizing the effect (Doppler effect) that the frequency of scattered light from the object irradiated by laser light Shifts in proportion to the moving speed of the object.
FIG. 1 is a diagram illustrating a conventional laser Doppler velocimeter.
In FIG. 1, a laser light beam emitted from a laser 1 becomes a parallel light beam 3 after passing through a collimating lens 2, and is divided into two light beams 5a and 5b by a beam splitter 4. After being reflected by mirrors 6a and 6b, the light beams 5a and 5b are projected onto an object 7 to be measured, moving at a speed V, at an indicent angle .theta..
Scattered light from the object 7 is detected by a photodetector 9 after passing through a condenser lens 8. At that time, the frequencies of scattered light beams of the two light beams are subjected to Doppler shifts of +.DELTA.f and -.DELTA.f in proportion to the moving speed V. If the wavelength of the laser light beam is represented by .lambda., .DELTA.f is given by the following expression (1): EQU f=V sin .theta./.lambda. (1).
The scattered light beams subjected to the Doppler shift of +.DELTA.f and -.DELTA.f interfere with each other, thereby producing a pattern comprising light and dark portions on the photosensing surface of the photodetector 9. The frequency of the pattern (hereinafter termed a "Doppler frequency") is given by the following expression (2): EQU F=2.DELTA.f=V sin .theta./.lambda. (2).
Accordingly, by measuring the Doppler frequency F of the output signal of the photodetector 9, the moving speed of the object 7 can be obtained from expression (2).
As is apparent from expression (2), in the above-described laser Doppler velocimeter, the Doppler frequency F is inversely proportional to the wavelength .lambda. of the laser light beam. Hence, it is necessary to use a laser light source capable of performing continuous oscillation and having a stable wavelength. A gas laser, such as a He-Ne laser or the like, is frequently used as such a laser light source. However, a gas laser oscillator is, in general, large in its size, and high voltage is required for its power supply, therby increasing the size and the cost of the entire apparatus. Although laser diodes (semiconductor lasers), used in compact discs, video discs, optical-fiber communication and the like, are very small and easy to drive, they have a problem of having temperature dependency of their characteristics.
FIG. 2 is a diagram illustrating a typical temperature dependency of the peak oscillation waylength of a laser diode, cited from "'87 Mitsubishi semiconductor data book; optoelectronic semiconductor devices". In FIG. 2, regions where the wavelength continuously changes reflect the temperature dependency of the refractive index of the active layer of the laser diode. The wavelength changes with a rate of 0.08-0.08 nm/.degree.C. in these regions. Regions where the wavelength discontinuously changes are called "vertical mode hopping" regions. The wavelength changes with a rate of 0.2-0.3 nm/.degree.C. in these regions.
In order to stabilize the wavelength of a laser diode, a method of controlling the laser diode at a constant temperature is adopted. In this method, temperature control members, such as a heater, a radiator, a temperature sensor and the like, must be mounted on the laser diode with a small heat resistance, and precise temperature control must be performed. As a result, the size and the cost of the obtained laser Doppler velocimeter considerably increase. Furthermore, instability caused by the above-described vertical mode hopping cannot be completely removed.
In order to provide a laser Doppler velocimeter which solves the above-described problems, a method has been proposed in U.S. patent application Ser. No. 839,708 (U.S. Pat. No. 5,216,478). in this method, a laser light beam from a laser light source is projected onto a diffraction grating. Two light beams, i.e., a +n-order diffracted light beam and a -n-order diffracted light beam (n=1, 2, . . . ) except a 0-order light beam, from among obtained diffracted light beams are projected onto an object 7 to be measured with the same crossing angle as the angle between the two light beams, and scattered light from the object 7 is detected by a photodetector.
FIG. 3 illustrates an example of diffraction when a laser light beam I is projected onto a transparent-type diffraction grating I, having a grating pitch d, perpendicularly to the direction of grating arrangement t. The angle of diffraction .theta..sub.0 is given by the following expression (3): EQU sin .theta..sub.0 =m.lambda./d (3),
where m is the diffract:Lion order (0, 1, 2, . . . ), and .lambda. is the wavelength of the laser light beam.
The diffraction angle of the .+-.n-order light beams, except the 0-order light beam, is given by the following expression (4): EQU sin .theta.=n.lambda./d (4),
where n=1, 2, . . .
FIG. 4 is a diagram =illustrating two light beams, i.e., .+-.n-order light beams, projected onto an object 7 to be measured via mirrors 6a and 6b with an incident angle .theta..sub.0, Scattered light from the object 7 is incident upon a photodetector 9 via a condensor lens 8. The Doppler frequency F of the output of the photodetector 9 is given by the following expression (5) using the above-described expressions (2) and (4): EQU F=2V sin .theta..sub.0 /.lambda.=2nV/d (5).
As is apparent from expression (5), the Doppler frequency F does not depend on the wavelength .lambda. of the laser light beam I, and is inversely proportional to the grating pitch d of the diffraction grating 10. Since the grating pitch d can be sufficiently stable, the Doppler frequency F is only proportional to the speed of the object 7. The same result can be obtained even if a reflection-type diffraction grating is used as the diffraction grating 10.
FIG. 5 is a schematic diagram of a principal portion of an optical system according to U.S. Pat. No. 5,256,885. In FIG. 5, there are shown a laser diode 1, a collimating lens 2 for making a laser light beam emitted from the laser diode 1 into a parallel light beam 3, and an object 7 to be measured. The parallel light beam 3 is projected onto a diffraction grating 10. Convex lenses 11 and 12 have a focal length f, and are arranged so that the relationship a+b=2f is satisfied, where "a" is the distance between the diffraction grating 10 and the convex lens 11, and b is the distance between the convex lens 12 and the object 7. These components constitute a laser Doppler velocimeter 101. The laser light beam, whose wavelength .lambda. is about 0.68 .mu.m, from the laser diode 1 is made into the parallel light beam 3, having a diameter of 2 mm, after passing through the collimating lens 2. The parallel light beam 3 is projected onto the transparent-type diffraction grating 10, having a grating pitch of 3.2 .mu.m, perpendicularly to the direction of the grating arrangement, and emanates .+-.1order diffracted light beams 5a and 5b with a diffraction angle .theta.=12.degree.. By projecting the light beams 5a and 5b onto the convex lens 11 having a focal length f of 15 mm, light beams 13a and 13b shown in FIG. 5 are obtained.
By projecting the light beams 13a and 13b onto the other convex lens 12 separated from the convex lens 11 by 2f (=30 mm), parallel light beams 14a and 14b are obtained. Each of the parallel light beams 14a and 14b has a spot size of 2 mm, and is projected onto the object 7, moving in the direction of an arrow 7a a at a speed V (mm/sec), with the same angle as the diffraction angle .theta. from the diffraction grating 10.
Scattered light from the object 7 is efficiently condensed onto the photosensing portion 9a of a photodetector by the convex lens 12 and a condenser lens 8. An arithmetic detector 14, which receives photosensing signal from the photodetector 9, detects the speed V from the Doppler frequency F using the following expression (6): EQU F=2V/d (6).
By making a=10 mm, and b=20 mm so that b is relatively long, the working distance can be designed to have a large value, and therefore the degree of freedom when installing the laser Doppler velocimeter increases.